The invention relates to a device for recombination of hydrogen and oxygen.
A device of this kind, to be described later in greater detail, is known from U.S. Pat. No. 4,911,879 (Heck et al.).
An apparatus of that nature is discussed in German Patent No. DE-A-36 04 416 (corresponding to the Klatt et al U.S. Pat. No. 4,755,359). As set forth in detail in the Klatt et al. patent, the problem of eliminating hydrogen from a gas mixture arises in particular in nuclear reactor accidents, in which hydrogen escapes into the oxygen-containing atmosphere of the containment vessel or a pressure suppression system of the nuclear reactor, thus creating the risk of an explosion. To avoid this explosion danger, known methods are employed to eliminate the hydrogen through catalytically supported recombination with oxygen to form steam. Especially suitable catalyst materials for this purpose and hence also within the scope of the present invention are described in German Patent No. DE-A-37 25 290. Since a catalyst of this kind forms part of the safety equipment, which is only supposed to operate in the event of a malfunction, care must be taken to ensure that the catalyst retains its functional ability over very many years of storage. For this purpose, methods are known in which the catalyst is stored in an airtight sealed housing, within the vessel or space in which the hydrogen is to be eliminated in the event of an accident, said housing opening automatically when the accident occurs as a result of the influence of pressure and/or temperature, thus exposing the catalyst to the atmosphere-containing hydrogen and oxygen.
During a core meltdown in a reactor pressure vessel (RPV), a temperature rise in the melt of up to 2400.degree. C. is reached, with large quantities of fission products and structural materials being released into the atmosphere of the containment. This results in a mixture of steam and gases in which aerosol particles with a weight concentration of up to 20 g/m.sup.3 can be suspended. The term "aerosol" is used herein in a broad sense to mean a suspension of liquid or solid particles in a gas. Thus for example in the low-pressure path at the beginning of the interaction between the melt and the concrete, 1 to 3 tons of dispersed material can be suspended in the air inside the containment vessel. By far the largest component, more than 95%, is non-radioactive. However, most of the radioactive substances are bound to the aerosol particles. The release of hydrogen during reactor accidents, mentioned at the outset, coincides in time with the above release of aerosols.
Model tests have shown that the release of steam occurs practically simultaneously with the beginning of a core meltdown accident, while the release of hydrogen and simultaneously therewith, the release of aerosols, take place only after a certain delay. In the presence of large quantities of steam and a strong flow, the catalytic reaction to remove hydrogen proceeds more slowly. The reaction rate increases exponentially with temperature. It is only when a sufficiently high temperature has been reached on the surface of the catalyst system that a sufficient convection flow develops which is adequate to prevent the aerosol particles contained in the gas mixture from being deposited on the surface of the catalyst. This prevention is aided by the constant generation of reaction steam at the surface of the catalyst system, which becomes constant at a correspondingly high temperature and conversion rate. However, as long as the temperature of the catalyst system is still not sufficiently high during the initial phase, aerosol particles and grease particles contained in the steam can settle on the surface of the catalyst, thus reducing the effective catalyst surface and having a highly negative effect on catalytic reaction.
Heck et al. mentioned at the outset, contains a catalyst system inside a cylindrical tube whose two ends are closed off by seals which open automatically in the event of an accident. The tube is mounted vertically in the area to be protected and has a filter system between its lower end and the catalyst system for chemically neutralizing catalyst poisons. The filter system can be a porous ceramic body or a molded fiber structure containing silver nitrate. When the seals at the two ends of the tube open, the atmosphere containing hydrogen penetrates the tube and passes through the filter into the catalyst system, which heats up because of the exothermic reaction, thus generating a gas flow through the tube.
Examples cited in Heck et al. of seals which open automatically as a function of temperature are diaphragms made of a plastic which melt at high temperatures, as well as bimetallic sheet metal. The bimetallic sheet metal has no gas-tight seal. On the other hand, plastic diaphragms do not provide reliable long-term gas-tight seals. In addition, in the event of ignition, they can burn and impose a burden on the environment through the release of gases.
The steam released initially in the event of an accident, in accordance with the above statements, passes through the rooms of the installation in which circulating pumps, slide bearings, electric motors, etc. are located, thereby carrying with it certain amounts of lubricating and sealing grease. Grease particles that reach the catalyst system can settle out on the catalyst surface, provided their temperature is below the vaporization point of the grease. It has been found that grease deposits of this kind have a highly disadvantageous effect on the action of the catalyst. Even a small amount of grease, only 0.05 g of grease per liter of steam, can prevent the catalytic reaction. To avoid the problems created by the grease, German Patent application P 40 03 833.5, not published previously, describes a protective device for the catalyst system. This protective device essentially consists of filters which are permeable to gas but have a high separation efficiency for aerosols and grease particles. The filters are so-called HEPA (High Efficiency Particulate Air) filters. These filters are made of glass wool and a binder which are highly temperature-resistant (up to about 900.degree. C.). The filters surround the catalyst system in such a way that aerosols and grease particles are kept away from the catalyst surface, while still permitting hydrogen and oxygen to reach this surface. As a result of inclusion by the filter and a correspondingly low heat loss, the temperature of the catalyst surface quickly rises because of the exothermic recombination reaction. As soon as the temperature has reached a value at which grease particles and aerosols can no longer settle on the catalyst surface, the filters open, thus exposing the catalyst system to unimpeded access by the atmosphere of the room to be protected, so that the catalyst system can then produce its total effect. The filters described in that patent application protect the catalyst system in the initial phase of an accident before aerosols and grease particles are deposited, however they cannot prevent the long-term deterioration of the catalyst as a result of catalyst poisons contained in the ambient atmosphere of the vessel, during the storage period prior to an accidental meltdown.
The operating time of a reactor is up to forty years. During this long period of time, the devices for recombination of hydrogen and oxygen must maintain total functional ability in a state of readiness. It is known that palladium and platinum as catalyst materials are sensitive to surface contamination and lose their effectiveness. The alloys described in DE-A-37 25 290 are less sensitive, but no results are available on long-term tests on the effects of impurities such as chlorine, sulfur, and the like.